In recent years, among photoelectric conversion devices, a thin film photoelectric conversion device that needs fewer raw materials has attracted attention and its development has been vigorously made in order to achieve both lower cost and higher efficiency of photoelectric conversion devices including solar cells. In particular, a method for forming a satisfactory semiconductor layer on a low-priced substrate such as glass by means of a low-temperature process has been expected as a method capable of realizing low cost.
Generally, in forming a superstrate-type thin film photoelectric conversion device, it is essential that a transparent electrode layer be provided in a part of the conversion device. The thin film photoelectric conversion device includes one or more photoelectric conversion units between the transparent electrode layer and a back electrode layer. Light is made incident from the transparent electrode layer side.
The transparent electrode layer is, for example, formed using a transparent conductive oxide such as tin oxide (SnO2) or oxide zinc (ZnO) by methods such as a chemical vapor deposition (CVD) method, sputtering method or vacuum deposition. It is desirable that the transparent electrode layer for use in the thin film photoelectric conversion device has fine irregularities on its surface so as to have the effect of increasing scattering of incident light. This is because, although a thickness of a photoelectric conversion layer in the thin film photoelectric conversion device can be made small as compared with that in a conventional photoelectric conversion device using bulk monocrystalline or polycrystalline silicon, the thin film photoelectric conversion device has a problem of light absorption of the entire thin film being limited by the film thickness. Thereat, in order to effectively use light incident on the photoelectric conversion unit including the photoelectric conversion layer, a method has been devised in which the surface of the transparent electrode layer or the metal layer in contact with the photoelectric conversion unit is roughened (textured), light is scattered on its interface, and then made incident into the photoelectric conversion unit, to extend an optical path length for increasing a light absorption amount inside the photoelectric conversion layer. This technique is referred to as “light confinement”, and has been an important element technique in putting the thin film photoelectric conversion device with high photoelectric conversion efficiency to practical use.
The photoelectric conversion unit is made up of a semiconductor layer of pn junction or pin junction. In the case of using the pin junction in the photoelectric conversion unit, the junction is formed by stacking a p-type layer, an i-type layer and an n-type layer in this order or in a reversed order thereof, and one whose i-type photoelectric conversion layer occupying its main part is amorphous is referred to as an amorphous photoelectric conversion unit, and one whose i-type layer is crystalline is referred to as a crystalline photoelectric conversion unit. For the semiconductor layer, there is used amorphous silicon or thin film crystalline silicone as a silicon-based thin film semiconductor, or CuInSe2 (abbreviated as CIS), CdTe or the like as a compound semiconductor.
In the silicon-based thin film photoelectric conversion device as an example of the thin film photoelectric conversion device, the pin junction is used for the photoelectric conversion unit, the junction being made up of the p-type layer, the i-type layer as a substantially intrinsic photoelectric conversion layer, and the n-type layer. Among them, one using the amorphous silicon for the i-type layer is referred to as an amorphous silicon photoelectric conversion unit, and one using silicon containing a crystalline material for the i-type layer is referred to as a crystalline silicon photoelectric conversion unit. It is to be noted that, as for an amorphous or crystalline silicon-based material, there is not only a case where silicon is used alone as a main element constituting the semiconductor, but there can also be used an alloy material containing an element such as carbon, oxygen, nitrogen or germanium. Further, as a main constitutional material for the conductivity-type layer, one homogeneous to the i-type layer does not necessarily need to be used, but for example, amorphous silicon carbide may be used for the p-type layer of the amorphous silicon photoelectric conversion unit, and a silicon layer containing a crystalline material (also referred to as microcrystalline silicon) may also be used for the n-type layer.
As the back electrode layer formed on the photoelectric conversion unit, for example, a metal layer such as a layer made of at least one or more metals selected from Ti, Cr, Al, Ag, Au, Cu and Pt, or made of an alloy of these metals, is formed by a sputtering method, vacuum deposition method, or the like. Further, there may be formed a layer made of a transparent conductive oxide such as indium tin oxide (ITO), SnO2 or ZnO between the photoelectric conversion unit and the metal electrode.
As a method for improving conversion efficiency of the thin film photoelectric conversion device, there has been known a thin film photoelectric conversion device adopting a structure referred to as a stacked type (tandem type) in which two or more photoelectric conversion units are stacked. In this method, a front photoelectric conversion unit including a photoelectric conversion layer with a large optical band gap on the light incident side of the thin film photoelectric conversion device is arranged, and therebehind a back photoelectric conversion unit including a photoelectric conversion layer with a small band gap is sequentially arranged, to allow photoelectric conversion over a broad wavelength range of incident light, whereby incident light is effectively used so as to improve conversion efficiency as the entire conversion device. Among the stacked-type thin film photoelectric conversion device, one stacked with the amorphous photoelectric conversion unit and the crystalline photoelectric conversion unit is referred to as a hybrid-type thin film photoelectric conversion device. (In the present application, the photoelectric conversion unit arranged relatively on the light incident side is referred to as the front photoelectric conversion unit, and the photoelectric conversion unit adjacently arranged on the side farther than the front photoelectric conversion unit from the light incident side is referred to as the back photoelectric conversion unit).
In the case of manufacturing such a thin film photoelectric conversion device as a large-area thin film photoelectric conversion device capable of producing a high output at a high voltage as electric power, it is not that a plurality of thin film photoelectric conversion devices formed on a substrate are connected in series by wiring and then used, but generally, for obtaining good yields, a thin film solar cell formed on a large substrate is divided into a plurality of cells and those cells are connected in series by patterning so as to be integrated. In particular, in the thin film photoelectric conversion device using a glass plate as a substrate and making light incident from the glass substrate side, each cell is generally connected in series in a direction perpendicular to a longitudinal direction of the strip shape so as to be integrated thereby reducing a loss due to resistance of the transparent electrode layer on the glass substrate, wherein the integration is achieved by forming separation grooves by laser scribing which process the transparent electrode layer in strip shape with a predetermined width.
An integrated-type thin film photoelectric conversion device is generally produced by such a method as below. First, on a transparent substrate, a transparent electrode layer made of a transparent conductive oxide such as SnO2 or ZnO is formed, and the transparent electrode layer is subjected to scribing with a laser or the like. Next, a photoelectric conversion layer made of amorphous or crystalline silicon, or the like, is formed on the transparent electrode layer by a plasma enhanced CVD method, and thereafter, the photoelectric conversion layer is subjected to scribing with the laser or the like. A scribe line formed in the photoelectric conversion layer by this separation groove is used as a connection line for connecting a back electrode layer and the transparent electrode layer between two mutually adjacent cells. Subsequently, the back electrode layer made of a light reflective metal is formed on the photoelectric conversion layer by a vacuum deposition method, sputtering method or the like.
In such an integrated-type thin film photoelectric conversion device, the larger current and output power are desirably obtained. While, at the time of producing the conversion device by scaling up its area for cost reduction, what is important has been how current values generated in a plane can be made uniform and maximal by improving uniformity of a film thickness and film quality of each layer constituting the conversion device. The present invention provides a thin film photoelectric conversion device that improves nonuniformity of current values of photoelectric conversion cells caused by nonuniformity of a film thickness and film quality of a photoelectric conversion semiconductor layer, which occurs at the time of scaling up an integrated-type thin film photoelectric conversion device, to maximize output characteristics thereof.
For example, in Patent Document 1, a film thickness distribution of the photoelectric conversion layer formed by a plasma enhanced CVD method is caused by a gas flow rate distribution and an electric field distribution inside the device, and consequently, a current value in a portion with a larger film thickness is larger, and a current value in a portion with a smaller film thickness is smaller. As a technique for solving such in-plane nonuniformity of current values of photoelectric conversion cells, there has been disclosed a technique where a width of each string of the photoelectric conversion cells which was formed by laser scribing is adjusted to set constant current values that are generated, so as to improve output characteristics of the thin film photoelectric conversion device.